St Fergus (d. 583) is named as first Bishop of Down. The diocese of Connor was founded in 480 by St Macnissi, and St Malachy was bishop there (1124). The dioceses of Down and Connor were permanently joined in 1439.

In 1670, as an effect of the Reformation, wars, and penal laws, in the whole of Down and Connor there were only 2500 Catholic families. When at length the pressure of penal legislation was removed Catholicism revived rapidly.

In the period 1810–1840, due in part to Catholic emancipation in 1829, forty new churches were built. The progress thus made under Dr Crolly (1825–1835) and Dr Denvir (1835–65) was continued, as Belfast expanded as a city, under Dr Dorrian (1865–86) and Dr MacAlister (1886–95) and Dr Henry (1895–1908).

There are currently 88 parishes and ministries in the diocese served by 109 active priests. The Most Reverend Noel Treanor is the 32nd Bishop of Down and Connor. He was ordained to the Episcopate and installed as Bishop of Down and Connor on 29 June 2008.

At the Diocesan Congress in 2013 Bishop Treanor launched a review to ensure pastoral activities within the diocese matched current resources. To that end a review of pastoral structures was initiated which drew on the 2011 census. Draft new pastoral communities were presented to the clergy in three meetings and then to the lay faithful. Significantly influencing the structures for the future was a regular practice rate across the diocese of 20% which suggested a greater sharing of personnel, skills and finances.

Bishop Treanor joined other Irish bishops in February 2017 for the ad limina visit. [2]Unlike previous visits there were no private meetings with diocesan bishops and Pope Francis, rather the latter spoke with the bishops together. [3]